Welcome to the boards Josh... There are a couple of conditions that could cause swelling of the brisket. The most common is hardware disease. This is caused by ingesting some form of metal, usually a nail, screw, or piece of wire. The metal settles in the reticulum and eventually penetrates the reticular wall and diaphragm, The rumen contractions eventually cause it to progress until it begins to irritate the sac around the heart. I would strongly recommend having a vet check this heifer out. If caught early enough, there are some things that can help improve the situation. A rumen magnet can help pull the metal back into the reticulum and relieve the irritation to the heart. Anti inflammatories and antibiotics will also be required.
I hope this is helpful, and good luck.